The present invention is directed to a method of controlling the basis weight of a sheet material being manufactured from a liquid stock having a highly variable consistency or having an open stock tank.
There are many sheet products made from a liquid stock. Examples are: paper, various grades of paper board, asbestos roofing, gasketing, and ceiling tile. The machines used to make these products have a sheet forming apparatus to which a slurry of fiber and water is supplied. The water drains away and leaves a fiber sheet material. The fiber water slurry, often called liquid stock, is applied to the sheet former at a closely controlled rate. This is accomplished by allowing the stock to flow out of a tank at a certain speed which is, of course, dependent upon the total head in the tank. In many instances, the tanks are closed and the head is controlled by maintaining the liquid at a constant level and varying the air pressure above the liquid. However, in some instances, the tanks are open and the pressure or total head is determined by varying the level of liquid. With an open tank arrangement, the level is maintained by a variable overflow gate valve. However, this is unsatisfactory in that the gate is difficult to adjust even manually and close to impossible to adjust by automatic means.
In some installations, the percentage of fiber in the water, typically called consistency, varies widely when the level in the tank gets below a critical level. When this happens, the process becomes very difficult to control and the finished product is of poor quality. There are some installations where the consistency is constant, and the tank is closed but where there is no flowmeter on the stock lines supplying the tank. In these situations, current techniques for controlling the process, such as by estimating the flowrate in the stock line by measuring the valve setting, have proven to be very unsatisfactory.
Finally, as discussed above, because of the large variation in consistency, it is also desirable to automatically control consistency.